text: It is expected that some continual improvement of experimental sensitivity will allow discovery of very mild radioactivity (instability) of some isotopes that are considered to be stable today. For example, in 2003 it was reported that bismuth-209 (the only primordial isotope of bismuth) is very mildly radioactive, with the half-life time of (1.9 ± 0.2) × 10 yr, confirming earlier theoretical predictions from nuclear physics that bismuth-209 would decay very slowly by alpha emission.
  Isotopes that are theoretically believed to be unstable but have not been observed to decay are termed as observationally stable. Currently there are 105 "stable" isotopes which are theoretically unstable, 40 of which have been observed in detail with no sign of decay, the lightest in any case being Ar. Many "stable" nuclides are "metastable" inasmuch as they would release energy if a radioactive decay were to occur, and are, in fact, expected to undergo very rare kinds of radioactive decay, including double-beta emission.
  146 nuclides from 62 elements with atomic numbers from 1 (hydrogen) through 66 (dysprosium) except 43 (technetium), 61 (promethium), 62 (samarium), and 63 (europium) are theoretically stable to any kind of nuclear decay—except for the theoretical possibility of proton decay, which has never been observed despite extensive searches for it—and spontaneous fission, which is theoretically possible for the nuclides with atomic mass numbers ≥ 93.
  For processes other than spontaneous fission, other theoretical decay routes for heavier elements include:
  * alpha decay – 70 heavy nuclides (the lightest two are cerium-142 and neodymium-143)
  * double beta decay – 55 nuclides
  * beta decay – tantalum-180m
  * electron capture – tellurium-123, tantalum-180m
  * double electron capture
  * isomeric transition – tantalum-180m
  These include all nuclides of mass 165 and greater. Argon-36 is presently the lightest known "stable" nuclide which is theoretically unstable.
  The positivity of energy release in these processes means that they are allowed kinematically (they do not violate the conservation of energy) and, thus, in principle, can occur. They are not observed due to strong but not absolute suppression, by spin-parity selection rules (for beta decays and isomeric transitions) or by the thickness of the potential barrier (for alpha and cluster decays and spontaneous fission).